So my buddy has a '98 Sportstar nineteen, and it's got the TBI 305...which runs great and pulls great --- when it feels like it. Then it decides to start acting up --- and eventually just chugs out and pretty much dies, backfiring in the engine compartment. Usually, stopping by putting it in neutral, letting it idle for a minute will make it start running fine again. Sometimes it seems as if the spark isn't advancing at the same rate as the boat is trying to go, and it seems to misfire a tiny bit and choke out more, but then sometimes it just goes fine all day.

Boat and maintenance details: 650 hours, regular oil changes, spark plugs and wires have less than 100 hours...Just replaced: Fuel filters, fuel line, knock sensor, distributor cap and rotor, throttle position sensor, fuel pump, and the gas tank was removed, cleaned and replaced. It ran absolutely amazing for a week, then it started acting up again. The old computer was replaced at roughly 500 hours because it completely stopped working, and the boat worked fine with the replacement computer for the rest of that summer. This is the third summer that we've been dealing with this stumble and we really do not know what to do. The mechanic does not know what to do, corporate Mastercraft was no help on the phone aside from saying that it is a "really old boat" and they don't carry parts anymore and telling us to replace the fuel pump (which we did).

Anyone have any suggestions of where to start? We don't know if it's a fuel supply issue or an ignition issue...but I'm afraid to look at the throttle body with the spark arrestor removed to check for fuel flow because it's backfiring! I have one video of it backfiring, and another one I'm trying to upload that shows it stumbling a lot more from in front of the engine http://youtu.be/g3xPUPyomOU it's a terrible quality video so please excuse it, the backfire happens in the first ten seconds. I'll try to get the other video uploaded tonight...internet is being slow.

Well, here's the video where it screwed up again...badly --- it was running perfectly for 11 minutes of filming before this. It was running flawlessly for the previous 11 minutes before the start of the video and then it just slows down... http://youtu.be/ZHxsNNDYmw8

Thanks again, the only check engine code was knock sensor which has since been replaced. Really appreciate any help from anyone! I'm getting to the point that I'm gonna bust out my Nautique or step-dad's Malibu because they just work and it's frustrating to not be able to ski here at beautiful and glassy coved High Rock Lake! Restore our faith in the Mastercraft please :-)